The genetic marker HLA-B27 is found in a high percentage of patients with spondyloarthropathic disease (ankylosing spondylitis, Reiter's syndrome, and related disorders) but the underlying basis for this association is not known. The studies described in this proposal will investigate the relationship between HLA-B27 and disease. Two experimental approaches will be used, the combination of which should yield a high probability of determining whether the HLA-B27-associated disease susceptibility factor is the B27 molecule itself or an allele at another locus that exists in strong linkage disequilibrium with HLA-B27. A population of HLA-typed individuals will be studied, including B27+ and B27- normal controls, B27+ and B27- patients with spondyloarthropathies, control patients with other rheumatic diseases, and selected families. Peripheral blood lymphocytes from these individuals will be subjected to two fundamental lines of experimentation: (1) The cells will be assayed for susceptibility to lysis by a panel of human cytolytic T lymphocyte clones, each specific either for an epitope on the HLA-B27 molecule or for a control antigenic determinant. Correlations will be made within the study population between B27-associated, cytolytic T cell defined epitopes and the presence or absence of B27-associated spondyloarthropathic disease. These experiments will test the hypothesis that a T cell-defined HLA determinant is more closely associated with disease susceptibility than is serologically defined HLA-B27. (2) Genomic DNA extracted from the cells will be digested with restriction endonucleases; the resulting DNA fragments will be electrophoretically separated, blotted, and hybridized by the method of Southern with one or more 32p-DNA probes specific for base pair sequences of the HLA-B locus. Correlations will be made within the study population between the observed restriction fragment length polymorphism and the presence or absence of B27-associated disease. A genomic clone encoding the HLA-B27 antigen from a normal individual will be isolated. If a disease-associated DNA restriction fragment is identified, it will be subjected to molecular cloning and its structure will be compared with that of the cloned B27 gene.